The present invention relates generally to the field of power steering systems for motor vehicles and, more particularly, is directed to an improved motor-driven power steering system having a stable output torque.
Power steering systems for motor vehicles are well-known in the prior art. Such systems generate an auxiliary steering force in accordance with steering by the driver and the force thus generated is transmitted to a steering gear which turns the wheels. Most of the power steering systems in use at the present time are actuated by a hydraulic drive. The hydraulic drive is equipped with a control valve, hydraulic cylinder and related structure which generates an auxiliary steering force by moving hydraulic fluid within the cylinder in accordance with steering by the driver.
A significant disadvantage of hydraulic power steering systems, especially in light of the trend toward smaller cars, is that the control valve, hydraulic cylinder and related structure are large in size. The hydraulic lines must also be formed with a small curvature to prevent pressure loss in the system. In the case of vehicles not having a large mounting space in the engine compartment, such as front wheel drive vehicles, these constraints make it difficult to mount hydraulic power steering systems. Hydraulic systems must also be effectively sealed to prevent fluid leakage. Thus, the maintenance and installation of hydraulic power steering systems is quite troublesome.
In order to avoid the above-identified problems, it has been proposed to use an electric motor as the driving means for power steering systems. While motor-driven steering systems represent a significant improvement over hydraulic systems, it has been found that such systems exhibit a phenomenon known as "hunting". Hunting is generally defined as the time delay experienced between the issuance of control commands to a system and the start of the system's response to those commands. Power steering systems known in the prior art, as well as non-assisted steering operations, are also prone to instability at low steering force levels when the steering operation is first initited. The instability is usually transmitted to the driver as an uncomfortable vibration of the steering wheel. Moreover, steering systems known in the prior art are very sensitive to the condition of the road surface. Rough road surfaces also cause vibration to be transmitted to the steering wheel. While it may be possible to reduce such vibrations by adding various dampening devices to the steering system, the responsiveness of the system to the driver's control would be adversely affected.
Accordingly, there is a need in the art for a steering system for motor vehicles, particularly for vehicles having a motor-driven power steering system, which is readily responsive to control commands by the driver with little or no delay and which operates in a smooth and stable manner over its entire range of operation without regard to road surface conditions.